Pipe leakage detection

ABSTRACT

A device and a method for pipeline leakage detection. A pipeline leak detector travels through the bore of a pipeline, applies a test pressure to a fluid contained within the pipeline, and measures the resultant rate of change of pressure in the pipeline by using back extrapolation of the test data to determine an initial pressure drop rate. The degree of leakage at a given position in the pipeline is then determined from the rate of change of pressure in the pipeline.

The invention relates to pipe leakage detection for use in fluidpipelines (e.g. natural gas).

BACKGROUND OF THE INVENTION

1. Field of the Invention

There is a need to test pipelines for leakage and to preferably be ableto do so whilst the fluid is actually flowing through the duct, so as toavoid interrupting the downstream supply or services.

2. Discussion of the Background

The present invention is concerned with providing a mechanism fortesting leakage such as that which may occur in pipe joints.

SUMMARY OF THE INVENTION

According to the invention there is provided a pipeline leak detectorcomprising means for travelling through the bore of the pipeline; meansfor applying a test pressure to the pipeline; means for measuring theresultant rate of change of pressure; and means for determining thedegree of leakage at a given position within the pipeline from the rateof change measurement.

Further according to the invention there is provided a method ofdetecting leakage in a pipeline comprising moving a device through apipeline to a desired location; applying a test pressure to thepipeline, measuring the resultant rate of change of pressure anddetermining the degree of leakage at that location from the rate ofchange measurement.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example with reference tothe accompanying drawings in which:

FIG. 1 shows an embodiment of the leak detecting pig;

FIG. 2 shows a schematic view of the leak detecting pig within anexisting pipeline, and

FIG. 3 shows a graph associated with the testing configuration.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The leak testing pig train 10 of FIG. 1 is shown within an existingpipeline 11 which incorporates a joint 12 where two sections abut. Thepig train 10 includes a leak pig 13 with a flexible central body portion(e.g. of flexible plastics material) capable of 1D bend passing andconnected to a valve and sensor module 27, regulator module 20 forregulating the test gas, an electronic control module 14 and umbilicaltermination and power regulator module 15, the latter being linked via atrailing umbilical cable 19 to a base station 22.

The base station connects to a computer 29 (e.g. a laptop pc). Thecontrol module 20 includes a gas regulator valve 18 which receives gasfor test purposes via the umbilical 19. The umbilical also providespower to the pig as well as control and data lines (digital). The pig istowed through the pipeline 11 via tow cone 16 by means of towline 17attached to a winch 40. The umbilical cable 19 will be fed over anencoder wheel 21 to indicate distance travelled, this information beingpassed to the computer 29 via basestation 22. The pig is automaticallywinched under computer control through the gas main pipeline 11 in astepwise manner to perform pressure decay tests at each step. The systemcan be configured to provide some overlap at each step to ensure fullchecking of the pipeline.

A portable power generator 41 provides power for the pig, the computer29 and basestation 22. A gas bottle 42 (e.g. natural gas) provides thetest gas for pressure tests to the pig via umbilical 19 which passesover drum 43.

The pig includes four circumferential seals 23-26 to provide an annulartest volume in the region between seals 23 and 26. The test volume isbetween two annular seal volumes bounded by seals 24 and 25. The gassupply is allowed to pass through the pipe for use, by the presence of ahollow central tube portion 28. The flexible body portion between seals24 and 25 allows relatively tight bends during insertion and travel tobe accommodated as do the control and regulation requirements into anumber of separate modules to allow 8 inch pipe testing.

The pig includes three sensors 30-32 which are spaced circumferentiallyaround the pig to detect the location of the pipe joint 12 as the pigtravels through the pipeline. These sensors can each comprise a smallmagnetic source with associated magnetic sensor (e.g. Hall effect).

The modules include microprocessors to provide a data link to and fromthe computer 20 via cable 19 and the on-board electronics will receivesensor information as well as control the pig operation. A batteryprovides the power source or an alternative source.

In order to carry out the leakage test operation an equalisation valve35 is provided which, when open, under electronic control allows testand seal volume pressures to be balanced. A precision differentialpressure sensor system 34 is provided to determine pressure drops duringdecay tests. The sensor for illustrative purposes is shown on pig 13,but in practice it will typically reside within the appropriate module27 and be linked to the pig 13 by small pipes to allow sensing to beachieved at the inner and outer volumes described below.

The mechanism associated with the leak tests is illustrated in theschematic of FIG. 2. The schematic drawing shows the pig 13 withinpipeline 11 and a leak being present in joint 12 (the leak beingexaggerated for illustrative purposes). The seals 24 and 25 togetherwith the outer wall of cylindrical pig portion 36 and the pipe wall forma first chamber 37 (when equalisation valve 35 is closed). The seals 23and 26 together with the inner wall of cylindrical pig portion 36 andthe wall of the inner cylindrical pig portion 38 form a second chamber39. Test gas will leak through the joint leak (Q leak). There may alsobe leakage (q seal) of the test gas between the chambers as these maynot form a perfect seal with the pipe. However, this is dealt with inthe computations.

The test volume is within chamber 37 and the seal volume is withinchamber 39 and with the valve 35 open their pressures are balanced. Fora perfect gas, $\frac{p}{t} \propto \frac{Q}{V}$

where Q is the flow out of volume V of chamber 37.

Using a test pressure, leakage can be detected and in practice thedegree of leakage can be measured as well.

A pressure decay test is performed by closing the equalisation valve andmonitoring test pressure using the high resolution pressure transducersystem 34. In practice, the main valve on closure can cause disturbancesdownstream. In order to produce a more stable reference, a small volumereference chamber is provided with its own regulation valve in series tokeep a stable reference value just during the main valve sequence.

Pressure decay due to a leak from the test volume will be reduced byleakage past the inner seals 24, 25, from seal to test volume. However,at the instant the equalisation valve 35 is closed, there is zerodifferential pressure across the inner seals and therefore no leakagepast them. Using back extrapolation of test data, it is possible todetermine the initial pressure drop rate dp/dt at the instant theequalisation valve is closed, and since the test volume is known,leakage Q can be calculated.

We have determined that even though there may be leakage into thepipeline from seals 23 and 26 as well and between chambers 37 and 39 dueto seals 24 and 25, it is the slope of the leakage curve that is relatedto the joint leakage.

Hence from FIG. 3 different graphs are shown for a given joint leak (Qleak) for various leakage patterns for the seal between chambers (qseal). Thus graph (e) shows the most effective seal and graph (a) theleast effective. Using calculus to determine the slope, $\frac{p}{t}$

is directly related to Q leak. The inner seal leakage is zero theinstant the equalisation valve is shut.

By using a relatively small chamber 37, the small test volume will givelarge drop rates for Q leak so detecting small leaks. Hence the pig willbe winched, in steps, under computer control. At each step, winching ispaused to allow the test to be effected. Leakage can occur at hot spotsor joints for example. Where leakage is at a joint, the presence ofdetectors 30-32 (of FIG. 1) will identify the source of leakage.

If a major leak is detected along the pipe at any location, this cancause an alarm or other indication in the P.C. as detected by beingunable to balance pressures in the test and seal volumes.

Typically leakage measurement is from 0.0028 SCMH (0.1 SCFH) to 1.0 SCMH(35 SCFH) in a low pressure main. Leakage measurement results can bewithin 10% accuracy or better.

Inner Seal Integity Test

In the event that the pig stops with an inner seal resting over anintrusion or debris, leakage past the inner seal may be such that testand seal volume pressures remain equalised when the equalisation valveis closed. This would mask any leakage from the test volume, if furthertesting does not occur. However, if the inner seals are functioningcorrectly, a forced increase in test volume pressure (e.g. by changingthe preset regulation pressure) with the equalisation valve closed wouldgive an increase in differential pressure across the inner seals. Bymonitoring this effect, seal integrity can be checked at each test stepalong the pipe. Alternatively, venting of the test volume via anothervalve to the actual pipeline pressure to achieve a pressure drop willalso serve as a mechanism for checking seal integrity

Service Location

If services are taken from the pipeline, it will be necessary todiscriminate between joint leaks and pressure drops due to consumptionat service pipe locations. Service pipe junctions could be detected by amagnetic source present in the service pipe, for example.

Leakage from services can be measured, if the service is blanked off inthe property. In this case, an additional deliberate leak at theproperty end of the service will be used for service volumequantification. If service leakage measurements are not required, theflow through facility on the pig ensures continuity of gas supply. Inthis case, leakage measurements while the pig is parked over the servicewould be masked by the demand from the property and would be discarded.

The computer provides a user interface for entry of site details, arunning graphical display of leakage versus distance along the main, andsoftware to drive the test sequence and winch control systems.

Post inspection data analysis will allow on site graphical or reportstyle presentation of inspection results, showing the position andmagnitude of leaks above a user set threshold, together with thepositions of joints and services.

Inspection time will typically be 20-30 minutes per 100 meters of main.Hence the pig is designed for use in live gas mains typically withoutinterruption to downstream gas supply or services. It incorporates jointand service position detection and will determine the position andmagnitude of leakage from mains and services.

The leakage pig is unique as it will both locate and accurately measuregas leakage from distribution pipes, dead or live. The source of leakagecould be a faulty joint or a pipe defect.

The pig is therefore able to:

1. accurately quantify leakage both for inspection purposes and forcollection of valuable leakage data,

2. test integrity of any in pipe repair; and

3. locate leaks where barholing and external repair is precluded.

The device has been described in terms of carrying out checks whilst itis temporarily stationary over any particular pipe position and utlisingthe equalisation valve, before the device moves forward again to thenext incremented step position.

However in a further arrangement the valve could be replaced by anequalisation aperture and the device could move continuously through thepipeline to carry out its tests.

This freeflow detection would be particularly suitable for testing smallleaks (e.g. of the order of 100 scmh) in a transmission pipeline system,by employing both local pressure drop measurements and pipe boremapping.

What is claimed is:
 1. A pipeline leak detector for conducting a leakagetest sequence comprising: means for travelling through the bore of thepipeline; means for applying a test pressure to a fluid contained withinthe pipeline; means for measuring the resultant rate of change ofpressure by using back extrapolation of test data to determine theinitial pressure drop rate; and means for determining the degree ofleakage at a given position within the pipeline from the rate of changemeasurement.
 2. A detector as claimed in claim 1 including means forcomputing the instantaneous rate of change of pressure (dp/dt) followingthe application of the test pressure.
 3. A detector as claimed in claim2 including differential pressure sensing means for determining thepressure difference between an applied pressure in a first chamberwithin the detector and a second chamber adjacent the first chamber,said second chamber forming a test chamber for leak detection to allowthe rate of change of pressure to be determined therefrom.
 4. A detectoras claimed in claim 3 including aperture means between the first andsecond chambers to allow equalisation of pressure to be provided priorto the leak measurement.
 5. A detector as claimed in claim 4 wherein theaperture means includes a valve for controlling when the aperture isopen.
 6. A detector as claimed in claim 1 including means forincrementing the travel along the pipeline and means for testing forleakage at that location.
 7. A detector as claimed in claim 6 whereinthe incrementing means is configured to cause the tests to be effectedin overlapping steps.
 8. A detector as claimed in claim 1 includingmeans for discriminating between joints and the remainder of thepipeline.
 9. A detector as claimed in claim 8 wherein the discriminatingmeans includes a magnetic sensing configuration.
 10. A detector asclaimed in claim 1 including umbilical means for providing a testpressure from a remote source.
 11. A detector as claimed in claim 10wherein the umbilical means includes an electrical cable.
 12. A detectoras claimed in claim 1 wherein the means for travelling through the pipeincludes an elongate body portion of flexible material.
 13. A detectoras claimed in claim 1 wherein the means for applying a test pressureinclude a first chamber formed between the pipeline wall and thedetector and a second chamber coaxial with the first chamber, and meansare provided to apply a fluid test pressure to the chambers from asource separate to any fluid flowing in the pipeline.
 14. A detector asclaimed in claim 13 including means for providing an aperture betweenthe first and second chambers to allow a pressure equalisation to beobtained.
 15. A detector as claimed in claim 14 wherein the aperture isconfigured to close as part of the test sequence.
 16. A detector asclaimed in claim 1 including means for determining the distancetravelled by the detector within the pipeline to assist in location ofthe position of a leak within the length of the pipeline.
 17. A detectoras claimed in claim 1 including means for detecting a pipe junctionwithin the main pipe.
 18. A method of detecting leakage in a pipelinecomprising: moving a device through a pipeline to a desired location;applying a test pressure to a fluid contained within the pipeline;measuring the resultant rate of change of pressure by using backextrapolation of test data to determine the initial pressure drop rate;and determining the degree of leakage at that location form the rate ofchange of pressure.
 19. A method as claimed in claim 18 including thestep of computing the instantaneous rate of change of pressure (dp/dt)following the application of the test pressure.
 20. A method as claimedin claim 19 including the step of determining the pressure differencebetween an applied pressure in a first chamber and a second chamberforming a test chamber for leak detection to allow the rate of change ofpressure to be determined therefrom.
 21. A method as claimed in claim 20including the step of equalising the pressure in the first and secondchambers prior to the measurement step.
 22. A method as claimed in claim18 including the step of detecting a joint within the pipeline as thedevice moves therethrough.
 23. A method as claimed in claim 18 includingthe step of discriminating between pipe junctions and pipe joints.